(1) Field of the Invention
The invention is related to a rotor system of an aircraft, particularly to a rotary wing aircraft lighter in weight than 1.5 ton or to rotary wing-unmanned aerial vehicles.
(2) Description of Related Art
Rotor systems for helicopters, especially those with high performance and low-noise characteristics, are extremely expensive concerning their development, manufacturing and certification. Especially their development implies a high risk potential concerning the helicopter's and rotor system's dynamic behavior. The manufacturing and certification of a rotor system are particularly expensive, when composite structures and dampers are applied in the rotor head.
The documents U.S. Pat. No. 4,131,391 A (Robinson) and U.S. Pat. No. 3,347,320 A (Cresap) disclose two-bladed rotors, connecting two, opposing rotor blades flap- and lag stiff with each other through a yoke to the rotor hub, where this yoke can teeter about an axis being perpendicular to the rotor mast and to both longitudinal rotor blade-axes as well.
The document U.S. Pat. No. 3,804,552 A (Covington) discloses helicopters traveling in level flight to be operating in a one “g” condition; that is, the rotor is producing lift equal to the vehicle weight. In a helicopter equipped with a gimbaled, teetering rotor, which cannot transmit a rotor moment into the mast head, a control moment about the aircraft center of gravity, a requirement to command a change in aircraft attitude, is obtained by tilting the rotor and hence its thrust vector. Thus, the control moment is a function of rotor thrust and tilt angle. The rotor is capable of producing a moment, if cyclic pitch is introduced to the rotor through a normal helicopter control system. This moment may be transmitted across a gimbal, down the supporting rotor mast to exert a controlling moment about the helicopter center of gravity by means of an elastomeric hub spring, connecting the gimbaled rotor hub to the rotor mast. This spring attaches to a first flange as part of the rotor hub yoke, and a second flange bolted to the supporting mast.
The document U.S. Pat. No. 4,695,227 A (Head) discloses a universal joint for use in a pitch cone rotor system in a helicopter comprising crossed lift beams, one beam of which is coupled to the rotor hub and the other beam of which is coupled to the driven rotor shaft. The two crossed lift beams are coupled to each other through a cruciform case. The cruciform case in turn is coupled to each of the crossed lift beams by means of a plurality of flexures. The flexures and cruciform case are made of composite fiber materials and the flexures are soft enough to permit angular and translational deflections of the crossbeams with respect to each other thereby tending to smooth out and reduce sudden changes in rotor speed between the shaft and rotor system and thereby tending to make rotor speeds more uniform. The crossed lift beams are further coupled together at the center of their crossing by a flexible elastomeric tension link which conducts rotor loads between the two beams when the rotor develops negative lift loads on the ground and in flight. A hollow laminated elastomeric compression fitting is placed between the beams at their center to transmit the rotor lift force between them.
Prior art information disclosure also includes the documents: US 2007/0084962 ZIENTEK, US2009/0214342 BRINDEJONC and US2012/0201678 HUNTER.